As more and more component assemblies and/or component parts become progressively more complex in increasingly smaller units, e.g., in semiconductor engineering, microsystems technology, telecommunications, security technology, energy production or measuring technology, the component assemblies and/or component parts are increasingly embodied by individual chips which are arranged adjacent to one another on a substrate.
Chips of this kind can have outer dimensions in the micrometer range.
Within the meaning of the present description, “substrate” designates a base material serving as a carrier for, e.g., printed circuit boards, circuits, conductor boards, lithography masks or photomasks, modules, e.g., for smartphones, for mobile communications or for chip-based security solutions, e.g., passports, for SIM cards, energy-efficient solutions, memory units, etc. However, it can also be a printed circuit board that is already pre-treated in a required shape or treated or outfitted on one side.
Conventional pick-and-place robots are typically used to populate a substrate with chips. These pick-and-place robots typically process components or chips having a thickness of several hundreds of micrometers and a surface area of several square millimeters to one square centimeter or more. They are not suitable for handling microchips, that is, within the meaning of the present application, chips with a surface area of at most a few square millimeters, particularly a surface area of less than 1 mm2 and thicknesses of less than 200 μm to less than 50 μm, e.g., 5 μm. Microchips with a surface area of less than 1 mm2 and a thickness of less than 5 μm are referred to hereinafter as membrane chips insofar as a microchip on this order of magnitude is specified. Aside from typical microchips with an electronic function, referred to hereinafter as electronic microchips, there are microchips with an optical function, referred to hereinafter as optical microchips. They can have even smaller dimensions than electronic microchips, particularly smaller thicknesses of only 1 to 2 μm. The surface area determined by the length and width of approximately 30 μm to 1 mm can be even smaller than in electronic microchips. Optical microchips are produced from optically transparent materials, e.g., SiO2 or Si3N4. Therefore, optical microchips in particular are constructed as membrane chips.
There are microchips and particularly membrane chips having dimensions of up to two orders of magnitude below those of conventional chips and, therefore, masses which are smaller by about six orders of magnitude. The small thickness and small surface area result in specific problems with regard to handling, precise positioning, planarity, stable connection and, as the case may be, conductive bonding of the microchip.
Microchips having such small geometric dimensions cannot be processed through conventional automatic populating devices, glue/solder connections, wire bonding or flip chip bonding. Common integrated circuit packaging methods such as sawing, pick-and-place, gluing, soldering, wire bonding or flip chip bonding can be resorted to only to a limited extent or not at all.
For example, while there are micro-manipulators for manipulating samples of focused ion beam (FIB) sections, they are very expensive to use and are not suitable for a fast and efficient component fabrication or microchip processing. There are also many micromechanical grippers documented in the literature, but they do not allow long movement paths and would have to be combined in a complicated manner with long manipulators having extensive reach.
EP 1 336 986 A1 discloses a method for producing a wafer with at least one thin electronic microchip which is held in a carrier body by a plurality of retaining tabs (referred to as fastening mechanisms) connecting the carrier body and microchip and a device and a method for separating an electronic microchip of this type from a wafer and for handling the removed microchip. By “microchip” is meant in this case chips having a thickness of less than 100 μm, especially a thickness of between 50 μm and 5 μm.
A prefabricated wafer with microchips which are held in each instance inside a cutout of a carrier body by a plurality of retaining tabs connecting the carrier body and microchip has the advantage that the microchips are ready to be transported after being severed and arranged on a substrate without having to undergo a further machining process.
To protect the wafer during transporting and handling and to remove the microchip, the wafer is placed on a base which has an arrangement of through-holes correlating with the cutouts of the carrier body so that the through-holes are arranged underneath the microchip to be separated. It is indicated as important that the base is rigid because the wafer is at risk of fracturing due to the plurality of microchips which are surrounded by a groove and connected to the wafer only via the retaining tabs. A free movement of one of the ejector pins in one of the cutouts of the carrier body and, therefore, positioning thereof beneath one of the microchips is made possible through the through-holes.
For separating a microchip, a handling device is advanced toward the microchip from the side of the wafer remote of the base. A downward pressure is applied to the microchip opposed to the ejector pin with this handling device. In so doing, the retaining tabs are flexed and broken. The microchip is preferably held on by suction during this so that it cannot slip when the retaining tabs break and is held horizontally.
After the microchip has been severed, the ejector pin and the handling device are moved upward together to ensure that remnants of retaining tabs do not strip off the microchip from the handling device. Subsequently, the ejector pin is lowered again and the microchip is held and transported such that it is sucked against the handling device by a vacuum.
In order to place the microchip on a substrate, a membrane in this instance, the handling device is positioned over the membrane and lowered. The suction tip provided at the handling device is spring-mounted so that it can be fitted softly and the membrane is not damaged. The vacuum source is switched off and the handling device is lowered and is ready to remove a subsequent microchip.
It is not shown how the microchip is fastened to the membrane.
It can be deduced that the circumferential shape of the microchip according to the above-cited EP 1 336 986 B1 is rectangular, i.e., the circumferential edge of the microchip is described by straight lines forming a rectangle.
In contrast to the above-cited EP 1 336 986 B1 in which the circumferential edge is defined by a slit which completely surrounds the electronic microchip and which is partially overlapped by retaining tabs which are arranged subsequently, it is also known to produce microchips which are held in a carrier body by retaining tabs which are monolithically connected with the microchip at this carrier body and which are formed by webs which interrupt the surrounding slit. A method suitable for this purpose for producing microchips with optical function is, e.g., anisotropic Si-etching in which SiO2 layers are processed to form membranes. The microchip is then severed from the wafer from the membrane by a cutting process along its circumferential edge up to the remaining retaining tabs in the form of webs. In other words, the portion of the membrane that forms the microchip remains monolithically connected to a portion of the membrane at the wafer. Up until the present time, however, microchips could only be produced in this way to a limited extent to thicknesses of greater than 5 μm because the methods for severing them were not suitable for smaller thickness.